4/15/12 Chapter 26: Properties of Light Field Induction Ok, so a changing magnetic field causes a current (Faraday s law) Why do we have currents in the first place? electric fields of the charges Changing current produces a magnetic field, too! (Maxwell s counterpart to Faraday s Law) Electromagnetism is a 2-way street Electromagnetic Waves An EM wave is the result of the mutual induction of electric and magnetic fields If the wave has wavelengths between ~400-700 nanometers, it will be a visible EM wave: Light! The speed of light is the wave speed for EM waves. Moving charges (currents) create magnetic fields, and changing magnetic fields create electric fields These effects create electromagnetic waves Recall: Properties of Waves Wavelength and Frequency Wavelength is the distance between two wave peaks Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency wavelength x frequency = speed of light = constant 1
Speed of Light c = 3.0 x 10 8 meters/sec in a vacuum Example: What is the wavelength of radio waves being broadcast at a frequency of 100 megahertz? The Electromagnetic Spectrum Gamma Rays X-rays Ultraviolet Light Visible Light (ROY G BIV) Infrared Light Microwaves & Radio Waves Light vs. Sound Important distinction: sound waves need a medium in order to travel. Light waves need no medium, they can travel even in a vacuum. How do light and matter interact? Reflection and Scattering Emission Absorption Transmission Transparent objects let light through Opaque objects block or absorb light Reflection or Scattering Mirror reflects light in a particular direction Movie screen scatters light in all directions 2
Light Waves in Matter Light travels at 3.0 x 10 8 m/s in a vacuum Light waves in matter travel more slowly, depending on the material and the frequency of the wave Slowing of light Dispersion High f = more dispersion Low f = less dispersion Transparent vs. Opaque Materials When light hits the atoms in a material, the electrons absorb it and are forced to vibrate The vibrating electron either emits a photon or transfers the energy as heat Time-delay between absorption and reemission is why the speed of light is lower in the material. Transparent Materials Average speed of light through different materials Vacuum: c (300,000,000 m/s) Atmosphere: slightly less than c (but rounded off to c) Water: 0.75 c Glass: 0.67 c, depending on material Diamond: 0.41 c Opaque Materials Most things around us are opaque; they absorb light without re-emitting it. Vibrations given by light to their atoms and molecules are turned into random kinetic energy (into internal energy). These materials become slightly warmer Materials can be opaque to some kinds of light and transparent to other kinds of light What about shiny metal objects? Recall that metal materials often have lots of free electrons These electrons can reflect the light, rather than transmitting it or absorbing it (more on reflection later) 3
A beam of light passes through a transparent sheet of plastic. Which of the following is true? A. The speed of the light slows down while in the plastic B. The frequency of the light is decreased while in the plastic C. Both A and B are true. Shadows Many everyday objects are opaque, so they block light. Shadows are the areas where light does not reach! Eclipses: Extreme Shadows In a solar eclipse, the Moon is precisely in between the Earth & the Sun, and casts a shadow on the surface of the Earth. In a lunar eclipse, the Earth is precisely in between the Sun & the Moon, and casts a shadow on the Moon Eclipses are relatively rare occurrences: most of the time, the Moon, Earth, & Sun are not lined up. Cornea How does your eye form an image? Refraction Focusing Light Refraction is the bending of light Eye uses refraction to focus light Refraction can cause parallel light rays to converge to a focus 4
4/15/12 Image Formation Rods & Cones Two different kinds of antennae that pick up light in our retinas Rods: handle vision in low light Cones: handle color vision and detail Three types of cones, sensitive to different frequency ranges. The focal plane is where light from different directions comes into focus The image behind a single (convex) lens is actually upside-down! Seeing and Perception Since only rods fire under low light, we mostly see in b&w when it s dark Edges of the retina are very sensitive to motion Our retinas have lateral inhibition so we can see details even under high contrast Our eyes highlight edges and differences (this is the source of many optical illusions) Main Points Light is EM waves The EM spectrum Transparent and Opaque materials Sight and Perception 5